The invention relates to apparatus for transferring fluid, and more particularly to a fluid transfer system which includes a pump that can be operated intermittently without priming.
In many pumping applications, the pump does not remain flooded during periods of nonuse. In many applications, starting the pump requires priming, bleeding of lines, or other steps to flood the pump and initiate flow of liquid therethrough. Steps such as priming often require manual intervention by a skilled operator.
One context in which the problem arises is in the use of liquid natural gas (LNG) as a fuel for motor vehicles. LNG is maintained typically at a temperature of about -250.degree. F. to -260.degree. F., and is typically maintained at equilibrium with its vapor in an insulated pressure vessel. For convenience in providing LNG fuel to vehicles, it may be desirable for the pump to be in an exposed, above-ground outdoor location which cannot efficiently be maintained at temperatures below the boiling point of the LNG while the pump is not in use. Centrifugal pumps have been found to be well suited for pumping of LNG, but require priming or other steps for start up when filled with vapor following a period of nonuse. In the context of fueling motor vehicles, it may be desirable for the pump to be usable by persons not particularly trained in start-up or other aspects of pump operation.
One method of avoiding pump start-up problems is to operate the pump continuously, controlling fluid flow through appropriate valving so that liquid may be dispensed on demand, and recirculated at other times. However, considerations such as energy efficiency and control of pump wear militate in favor of operating the pump only in response to a demand for transfer of fluid.
A general object of the invention is to provide a pump suitable for use in pumping fluids such as LNG on demand, without requiring continuous operation of the pump, and without requiring the services of a skilled technician to prime the pump, bleed vapor from lines, or otherwise take manual steps to flood the pump after a period of inoperation.
Among the problems which must be addressed in providing a fluid transfer system in this context are the effect of the low temperature liquid on pump components, both during normal operation and as a consequence of leakage. Cryogenic fluids tend to increase viscosity of conventional lubricants often used in pumps, leading to accelerated wear and other problems. It is an object of the invention to address the problems associated with handling cryogenic fluids such as LNG in pumping such fluids.